1. Field of the Invention
The present invention relates generally to a plating system of semiconductor assembly process, and, more particularly, to a rack for carrying a plurality of lead frames to be plated, which comprises indexing means for aligning said rack and a plating system using the same.
2. Description of the Prior Art
Semiconductor packages are electrically connected to external circuits through the outer leads of a lead frame. The electrical connection between the outer leads and external circuits is generally accomplished by soldering, and the surface of each outer lead made from Fe alloy or Cu alloy should be plated with tin or tin-lead alloy for the purpose of soldering. The surface treatment of outer lead of lead frame is accomplished through a redox reaction between a plating solution and each outer lead, in a plating bath. The surface treatment is usually performed by immersing a rack carrying a plurality of lead frames to be plated into a plating bath of plating machine.
FIGS. 1 and 2 show conventional plating machine. Plating bath (10) contains a plating solution, and a plurality of lead frames (14) to be plated is loaded into the body of a rack (12). Rack (12) is hung to a hoist flight bar (16) of the plating machine and fastened by a bolt. Hoist flight bar (16) serves as a plating electrode. The rack hung to the hoist flight bar is then transferred by a transferrer (18) to the plating bath (10), where the lead frame is plated.
For this apparatus, loading and unloading of rack to and from the hoist bar is performed by bolting and unbolting latch bars (20).
FIG. 2 is a side view of the apparatus of FIG. 1. Rack (12), provided with latch bars (20) each having a groove (21), i.e., a hook-forming slanted notch, is loaded into the plating machine by hanging it by means of the grooves (21) to the hoist flight bar (16) having projecting parts (22), i.e., horizontal pins, and then fastened by a bolt. The loading/unloading of the rack carrying a plurality of lead frames to/from plating machine was manually carried out. Considering that the weight of rack carrying a plurality of lead frame amounts about 10-13 kg and the loading/unloading of rack should be carried out an average of eight(8) times during 6 minutes, the clamping is very hard work for a worker. Therefore, automation of the loading/unloading of the rack in the plating process of the semiconductor assembly process has been strongly needed.
However, since the conventional rack is coated with epoxy resin except for the latch bars (20), it is difficult to control the dimension of the rack, which is essential for automation. Further, the bars (20) have a high ductility, causing a difficulty in balancing each bar. Further, the hoist flight bar is conductive, because it should serve as a conducting wire for plating lead frames, and consequently automation using electric energy cannot be designed and, therefore, has not been practiced.